separate analog and digital ground
Friend, a good book to this topic with clear explaination is "High-Speed Digital Design: A Handbook of Black Magic", Johnson and Graham. Generally, digital ground is the gound point for digital comppnents and analogue ground is the ground point for analogue comppnents. Isolation of these two ground points is very importance when design of mixed-signal system (eg system with digital circuit and analogue interfacing). The book has suggested some ways to make the grounding better from noise induction.